It is well recognized in the automotive field that it is desirable to install radio reception equipment and radio transceiver equipment with a minimum of damage to the vehicle and with a minimum of disturbance of the aesthetic impression sought to be conveyed by the vehicle designers. To that end, multiband antennas have been utilized; and, in addition considerable effort has been expended to devise a multiband antenna that does not reveal to possible thieves the character of presently-unusual radio equipment in the vehicle.
The advent of citizens' band radio spawned arrangements in which band separation impedance networks, either near the radio equipment for the respective bands or adjacent to the antenna, were coupled through a common transmission line for all bands to a single antenna element, sometimes called a compromise antenna element, for all bands of interest. Some examples of such arrangements can be found in the U.S. Pat. Nos. 4,037,177 to E. A. Tyrey, 4,095,229 to J. O. Elliott, and 4,228,544 to J. H. Guyton.
A variety of techniques have been taught for coupling networks adjacent to the antenna. U.S. Pat. No. 4,571,595 to J. P. Phillips et al. employs in a path common to all bands a tuning capacitor connected to the antenna elements per se and a transmission line tuning stub connected to the antenna ground plane. A parallel resonant (at the lowest frequency), coaxial cable feed device is used with a triband trap vertical antenna having a coaxial-resonant end feed in U.S. Pat. No. 4,217,589 to A. F. Stahler. A. Brunner et al. show in U.S. Pat. No. 3,945,013 a double omnidirectional antenna provided with a triaxial band separation arrangement for a two-identification-frequency radar and in which the outer conductor of an inner coaxial line serves also as the inner conductor of an outer coaxial line. In Brunner et al., a shorted quarter-wave low band transmission line section is incorporated into the triaxial structure to present a high impedance to a narrow frequency range of low band energy that might otherwise be diverted into the inner (high band) line outer conductor path to the high band radio equipment.